Permeation of individual gases through a low density polyethylene package film with CO2-N2 and CO2-O2-N2 mixtures was simulated with a simple mathematical model. Permeabilities of carbon dioxide were of the order of 10(-12) m3/s m2atm/m with CO2-N2 systems. Including oxygen in the system did not change carbon dioxide permeabilities. Permeabilities of oxygen were of the order of magnitude of 10(-9) m3/s m2atm/m with this system. When a CO2-O2-N2 mixture was humidified no substantial change was observed in CO2 permeability, but the O2 permeability decreased almost 10 times. These results and the mathematical model presented may help in the modified atmosphere package design to increase the shelf life of produce.